This invention pertains to improving the B.sub.0 homogeneity in high resolution (HR) MAS NMR, especially for liquids and semi-solids at high fields, using low-inductance saddle coils on cylindrical surfaces inclined at 54.7.degree. with respect to B.sub.0. Related NMR coils are described by Doty in U.S. Pat. No. 4,641,098, and numerous coils are reviewed by James Hyde in `Surface and Other Local Coils for In Vivo Studies`, Vol. 7, of The Encyclopedia of NMR, Wiley Press, 1996. A copending application discloses related coils using litz foil, and another copending application discloses novel sample cells for HR MAS. Hill discloses one method of geometric compensation using oval saddle coils for HR NMR in U.S. Pat. No. 4,563,648.
There have been numerous applications of Magic Angle Spinning (MAS) for line narrowing in solid samples for more than two decades. The solid sample is usually contained in a hard ceramic rotor with press-fit turbine caps machined from high-strength high-modulus plastics such as polyimides--see for example, U.S. Pat. No. 5,508,615 by Doty et al (note the extensive list of typographical corrections). The coil has traditionally been a multi-tuned solenoid, as shown by Doty in U.S. Pat. No. 5,424,625, although Cory et al in U.S. Pat. No. 5,539,315 have used a loop-gap resonator in combination with a solenoid. Another copending application discloses thermal buffering and susceptibility compensation to permit the use of transverse coils inside HR MAS solenoids, and this invention is directed toward improving B.sub.0 homogeneity of such.
The recent semi-solids MAS applications stem largely from the fact that spinning a cylindrically symmetric sample at the magic angle averages its susceptibility effects to zero. Moreover, spinning at the magic angle averages the inhomogeneities produced by static magnetic cylinders aligned with the magic angle to zero. Hence, high resolution may be obtained with magnetically inhomogeneous samples, such as tissues and semi-solids, and the inhomogeneities produced by the cylindrical portions of the stator, coil, and housing are inconsequential. This is particularly important for applications with limited samples. While the literature is replete with attention to compensation of the magnetism of rf coils for HR NMR, the capacitor magnetism problems have been largely ignored.
Kost et al and the above referenced copending litz-foil application disclose methods of improving the B.sub.1 homogeneity of the conventional slotted-resonator, the Alderman-Grant half-turn resonator, and other related coils. However, for small samples at very high fields, performance is limited primarily by the poor B.sub.0 homogeneity that comes from the proximity of the four chip capacitors traditionally used in the Alderman-Grant and related HT coils. In high-resolution NMR, these problems have been largely circumvented by using distributed capacitors with cylindrical symmetry, but this approach does not work well in HR MAS because of the severe space constraints imposed by the spinner. Magnetic compensation of the chip capacitors is only partially successful because of the temperature and field dependence of their magnetism.
Various circularly polarized birdcage resonators, as disclosed by Edelstein et al in U.S. Pat. No. 4,680,548, in which 8 or more capacitors are uniformly spaced around the rings at each end, are often used for large samples, and they have also been shown to be usable for samples of the size encountered in HR MAS. However, they are much less efficient and much more difficult to tune than the coils of the present invention for small MAS applications. Compared to single-turn or half-turn coils, current concentrations and hence peak conductor edge heating are greatly reduced in multi-turn transverse (Zens) coils. Also, their high inductance premits the capacitors to be well removed from the region of critical B.sub.0 homogeneity, but their high inductance causes severe high-voltage breakdown problems for high-power at high frequencies and limits the frequency at which they may be used.